In a conventional Time Division Duplex (TDD)-based or Frequency Division Duplex (FDD)-based Multiple Input Multiple Output (MIMO) antenna system, there are a small amount of antennae for a base station, and a User Equipment (UE) may measure the entire MIMO channel matrix. A pilot signal for measuring Channel State Information (CSI) is called as a reference signal in a Long Term Evolution (LTE) system, and it may include, e.g., a Common Reference Signal (CRS) and a Channel State Information Reference Signal (CSI-RS) for a downlink. Usually, each antenna port is configured to transmit one pilot signal. For example, in a Long Term Evolution Advanced (LTE-A) system, the base station may support at least four CRS antenna ports and eight CSI-RS antenna ports, and one antenna port may occupy a set of time-domain or frequency-domain resources. These antenna ports are orthogonal to each other in the time domain, the frequency domain or the code domain, so the UE may measure the corresponding CS, e.g., the MIMO channel matrix. Then, the UE may determine, on the basis of the measured CSI, an optimal Rank Indicator (RI), an optimal Pre-coding Matrix Indicator (PMI) and an optimal Channel Quality Indicator (CQI), and report the CSI to the base station.
For the massive MIMO technique, the base station may be provided with a large-scale antenna array (consisting of hundreds or thousands of antennae), so as to improve a spatial resolution and increase a system capacity. In the case that the antennae are arranged in a two-dimensional, uniform, rectangular array, it is possible to provide a very high three-dimensional (both horizontal and vertical) resolution. However, the channels for the massive MIMO system are arranged in a high-dimensional matrix, and during the practical use of the massive MIMO system in the FDD system, there is a huge challenge for the base station to acquire the CSI. On one hand, it may take a considerable amount of resource consumption for the FDD system to feed back such a high-dimensional MIMO channel matrix coefficient that meets a certain accuracy requirement. In order to reduce the feedback overhead, academics have presented some methods for efficiently compressing and encoding the high-dimensional MIMO channel matrix, e.g., a compressive sensing method. However, it is merely able for the compressive sensing method to reduce the feedback overhead. On the other hand, in the case of measuring the complete MIMO channel matrix, a large amount of pilot antenna ports for measuring the CSI may be provided, resulting in a considerable amount of resource consumption for transmitting the pilot signals. In order to reduce the pilot resource consumption, one method is to reduce the size of the MIMO channel capable of being detected by the UE, i.e., to reduce the dimension of the MIMO channel matrix. The industry has proposed a scheme where a plurality of antennae is virtualized to correspond to one pilot antenna port, so as to reduce the pilot resource consumption. However, the MIMO antenna array currently used in the industry is still of a small scale.
Hence, with respect to the massive MIMO system, a scheme for transmitting the pilot signal in various beam directions is proposed, so as to facilitate the UE to select an optimal beam. In this way, instead of measuring the channel matrix through the pilot signal, a spatial beam direction where each UE is located may be measured. Each antenna port (i.e., beam pilot port) corresponds to one spatial beam direction, and pilot symbols from the antenna port may be beam-formed and pre-coded and then transmitted through all or parts of the antennae, so as to form a beam in a corresponding spatial direction. However, because it is able for the massive MIMO technique to provide a very high spatial resolution, there may exist a huge quantity of spatial beam directions. In the case that one antenna port is configured for each spatial beam direction, it is still necessary to consume a large amount of resources.
In a word, in the related schemes for transmitting the pilot signals in various beam directions, there is a huge quantity of spatial beam directions, and in the case that one antenna port is configured by the base station for each spatial beam direction to transmit the pilot signal, a large amount of resources may be occupied.